The Oceanographic Products and Services Division (OPSD) of the National Oceanic and Atmospheric Administration (NOAA) "collects, analyzes and distributes historical and real-time observations and predictions of water levels, coastal currents and other meteorological and oceanographic data." This wonderful site explains the science behind the tides and changing water levels, gives historical background on tidal predictions and tidal machines (including several fascinating old photographs), and details the challenges of measuring water currents. The combination of engineering, history, and oceanography ensures that students and educators will learn much here. For access to tidal data, a tidal and current glossary, and an interactive tidal prediction page, users may follow links from the OPSD homepage.